Section 3.2.4.19.10 of the Canadian National Building Code states that damage to or disconnection of an audible signal device in a dwelling unit may not interfere with the ability of devices outside the dwelling unit to sound an alarm.
Existing line isolation devices designed to serve this function in systems using 70v audio signaling have significant reaction times, require the presence of audio on power-up to perform fault isolation, and require the presence of an audio signal to maintain isolation. These limitations make the existing designs unacceptable for use with amplifiers which have fault sensing capabilities.
Existing designs presently use Polymeric Positive Temperature Coefficient devices, otherwise known as PPTCs, to provide passive isolation of short circuit conditions on audio lines. These devices are thermal in nature, requiring the initial presence of high values of current on the audio line before any isolation can take place. The lower current values associated with 70v audio and the significantly long reaction times inherent in the nature of PPTCs make them unsuitable for use in this application.
The thermal nature of PPTCs also results in significant sensitivity to ambient temperature conditions. Trip and hold currents for PPTCs typically used in this application are derated as much as 25% over the 0 to 49° C. rated temperature range.